3.5. Cryopreservation
Simple cell and tissue preservation techniques have disadvantages including limited shelf-life, high cost, risk of contamination or generic drift [125]. A more tangable option is cryopreservation where cells are preserved by cooling them to low temperatures typically in liquid nitrogen (−196 °C). At such low temperatures, biological activities of the cells are effectivly stopped. These includes the biochemical reactions that would lead to cell death under normal conditions and damage caused by the formation of ice crystals. Cryopreservation provides a valuable means for storing cells and tissues for future use. However, certain drawbacks exist, including damage that occurs to the cells during the freezing and/or thawing processes and the need to culture the cells after thawing to ensure that they have recovered properly. Such drawbacks limit the value of cryopreserved cells, particularly in situations where it is desirable to use the cryopreserved cells immediately or shortly after they have been thawed. There exist several methods to deal which such problems. One alternative is to suspend the cells in an alginate solution prior to cryopreservation. The cells can then be encapsulated (see Section 3.2) after thawing and used for their desired purpose without the need to culture the cryopreserved cells. Alternatively, cells are entrapped in hydrogels before they are cryopreserved [126,127]. This method is based on the discovery that cryopreserved cells that have been thawed, immediately suspended in alginate, and after encapsulation remain viable and are ready to be used [128]. To obtain off-the-shelf availability, distribution and storage of constructs, sterility testing and quality control, preservation of cells and tissues is vital [129].